In many areas of the world, there are large deposits of viscous petroleum. Examples of viscous petroleum deposits include the Athabasca and Peace River regions in Canada, the Jobo region in Venezuela and the Edna and Sisquoc regions in California. These deposits are generally called tar sand deposits due to the high viscosity of the hydrocarbon which they contain. These tar sands may extend for many miles and may occur in varying thickness of up to more than 300 feet. Although tar sands may lie at or near the earth's surface, generally they are located under an overburden which ranges in thickness from a few feet to several thousand feet. Tar sands located at these depths constitute one of the world's largest presently known petroleum deposits.
Tar sands contain a viscous hydrocarbon material, which is commonly referred to as bitumen, in an amount which ranges from about 5 to about 16 percent by weight. This bitumen is usually immobile at typical reservoir temperatures. For example, at reservoir temperatures of about 60.degree. F., bitumen is immobile, having a viscosity frequently exceeding several thousand poises. At higher temperatures, such as temperatures exceeding 200.degree. F., the bitumen becomes mobile with a viscosity of less than 345 centipoises.
In situ heating is among the most promising methods for recovering bitumen from tar sands because there is no need to move the deposit and thermal energy can substantially reduce the bitumen's viscosity. Thermal energy may be introduced to tar sands in a variety of forms. For example, hot water, in situ combustion, and steam have been suggested to heat tar sands. Although each of these thermal energy agents may be used under certain conditions, steam is generally the most economical and efficient. It is clearly the most widely employed thermal energy agent.
Thermal stimulation processes appear promising as one approach for introducing these thermal agents into a formation to facilitate flow and production of bitumen therefrom. In a typical steam stimulation process, steam is injected into a viscous hydrocarbon deposit by means of a well for a period of time after which the steam-saturated formation is allowed to soak for an additional interval prior to placing the well on production.
To accelerate the input of heat into the formations, it has been proposed to drill horizontally deviated wells or to drill lateral holes outwardly from a main borehole or tunnel. Examples of various thermal systems using horizontal wells are described in U.S. Pat. No. 1,634,236, Ranney; U.S. Pat. No. 1,816,260, Lee; U.S. Pat. No. 2,365,591, Ranney; U.S. Pat. No. 3,024,013, Rogers et al.; U.S. Pat. No. 3,338,306, Cook; U.S. Pat. No. 3,960,213, Striegler et al.; U.S. Pat. No. 3,986,557, Striegler et al.; and Canadian Pat. No. 481,151, Ranney. However, processes which use horizontal wells to recover bitumen from tar sand deposits are subject to several drawbacks.
One problem encountered with use of horizontal wells to recover bitumen is the difficulty of passing a heated fluid through the horizontal well. During well completion bitumen will sometimes drain into the well completion assembly. This bitumen may block fluid flow through substantial portions of the horizontal well and thereby decrease heating efficiency.
Another problem which is encountered when using horizontal wells is that often the area stimulated is insufficient to make it economical to recover hydrocarbonaceous fluids from the reservoir or formation. Additionally, when horizontal wells are utilized in a water saturated bottom water zone, water coning often causes too much water to be produced with the hydrocarbonaceous fluids. Water coning is the phenomenum where water is drawn upwardly from a water-bearing portion of a formation into the oil-bearing portion about the well. Water coning causes free water to be produced in the well which results in a much higher water-to-oil ratio than would be the case without water coning. This higher water-to-oil ratio is undesirable and results in increased operating costs.
Therefore, what is needed is a method to thermally stimulate viscous hydrocarbonaceous fluids in a formation or reservoir which has limited native injectivity where a high water-saturated bottom zone is encountered.